Stimulated Raman Scattering (SRS) is an inelastic scattering process where the energy of an optical “pump” wave is transferred into a co-propagating “signal” wave, or a Stokes wave, within a gain medium. The total exchange of energy from the pump to the Stokes wave is material dependant, and is known as a Stokes shift. These gain mediums for SRS are often optical fibers and can be of any geometric, polarization, or index profile configuration. The threshold for the Raman process is a function of the inherent Raman gain and the geometry of the medium and the optical intensity of the pump wave. The generalized equation for the Raman threshold in an optical fiber can be described by Pth=(16 AEFF)/Gr LEFF. This is a well known equation established by Agrawal in Nonlinear Fiber Optics that describes the amount of peak power, with particular optical fiber geometries, required to achieve equal power distribution between the pump and the Stokes waves. When sufficient energy is transferred from the pump wave to the Stokes wave, the Stokes wave can serve as the pump for a secondary Raman process. This process continues as all waves propagate down the length of the optical fiber. Depending on the length of the fiber, the process can cascade out to many Stokes frequency shifts.
This application leverages claims found in U.S. Pat. No. 7,340,136, wherein a Raman laser/oscillator utilizing at least three pairs of reflectors is claimed. This patent does not explicitly claim the exact method of reflection that is used in the oscillator, and the patent only gives Fiber Bragg Gratings (FBG's) as example reflectors. FBGs are the primary method of reflection in fiber optics. The gratings are typically written into single mode, FBG performance varies greatly depending on modal content of the optical fiber. This makes FBGs difficult to engineer accurately and precisely for fibers with significant modal content. Very wide spectral bandwidths are difficult to achieve with a single FBG. A Dielectric coating is much less susceptible to angular and modal properties of a laser beam and can be designed for extremely wide bandwidths—realizing the use of a non-single-mode fiber for Raman lasers and amplifiers. The invention closes the gap between efficient, narrow, and broadband visible and infrared lasers using fiber optics and the need for high power visible and infrared sources.
There are currently no methods available in the commercial or academic sectors that provide efficient means to utilize the cascaded stimulated Raman scattering to convert a discrete pump wavelength to a specific desired wavelength without the extraneous conversion and significant emission of undesired intermediate frequencies that are required to achieve the final desired wavelength.
A need, therefore exists, to have more versatile, robust and efficient Raman lasers and amplifiers that can be achieved via standard laser architectures (solid-state, diode, fiber, dye or glass lasers), broadband emission of multiple wavelengths in conventional or unconventional laser frequencies, or broadband “white” light from a single fiber source.